Code Inspection: minor fixes & more unit tests:
- Fix bug: if one linter returns errors and another returns errors:[], the
no-errors linter is shown in the panel as an empty heading. (Bug doesn't
happen if it returned null instead; but both are valid ways of indicating
zero errors).
- Improve docs on register() - more concise, more detail on how to indicate
zero errors
- Add unit tests for the bug fixed above plus several more async linting
cases that were fixed late in PR #6530
- Fix mislabeled existing unit test for reject()ing async linters
- Fix stray whitespace that snuck into merge c3411bb7
Merge pull request #7650 from adobe/pflynn/lint-tests-fixes

Code Inspection: minor fixes & more unit tests